I hate to be the negative one here, but this feature seems uncharacteristically over-engineered.

For example, it appears to require much more work to setup a Rails engine than before. You used to be able to simply add a plugin that included a config/routes.rb file, and those routes would automagically be mounted in your main application. Is that still the case? Is there any reason to go through all these steps if you're not making a shared gem?

Last point: the syntax ofmount Uhoh::Engine => "/failures"
seems backwards to me. I would have expected:

mount "/failures" => Uhoh::Engine
instead, which seems a lot more intuitive and consistent.